Station for reducing gas pressure and liquefying gas

ABSTRACT

The invention relates to a station comprising an expansion turbine  12;  means for recovering mechanical work (G) produced during the gas pressure reduction in the expansion turbine; a cooling system ( 6 ) comprising compression means (C 1,  C 2,  C 3 ), condensation means ( 14 ) for liquefying gas (G 11 ) using the cold provided by the cooling system, means for recovering heat produced by the compression means (C 1,  C 2,  C 3 ) of the cooling system and means ( 10 ) for heating the gas upstream of the expansion turbine that are associated with the heat-recovery means.

The present invention relates to a station for reducing the pressure ofa gas and for liquefying the gas, particularly natural gas.

Thus, the field of the present invention is that of treatment of gases,particularly of natural gases, for the production of liquefied naturalgas.

Liquefied natural gas is used in different applications, It is mainlyused as fuel for vehicles, particularly transport trucks. The fuel oilgenerally used for such vehicles can indeed be replaced by pressurizedgas or liquefied natural gas, In comparison with the use of bottles ofpressurized gas, the use of liquefied gas offers an advantage in termsof volume and weight inasmuch as, on one hand, natural gas liquefied bycooling occupies much less volume than the same quantity of gaseousnatural gas, and on the other hand, the thermal insulation of thecryogenic tanks is much less heavy than the jacket of the gas bottles.The vehicles therefore have much more autonomy. Liquefied natural gas ismoreover a clean energy source, which limits the discharge of fineparticles such as soot, etc.

Liquefied natural gas can also be used for supplying small gas powerplants or for supplying small networks in villages.

Gas pipelines, or pipelines, are pipelines intended for transportinggaseous materials under pressure. The majority of gas pipelines conveynatural gas between extraction zones and consumption or exportationzones. From treatment sites of the gas fields or storage sites, the gasis transported at high pressure (from 16 to more than 100 bars) todelivery sites where it must be brought to a much lower pressure so thatit can be used.

For this purpose, the gas passes through pressure reduction stations inwhich the pressure of the gas is reduced by expansion through a valve ora turbine. The pressure reduction thus achieved produces energy which,in the case of a valve, is lost.

There are known gas expansion systems that use the natural gas enteringthe pressure reduction stations as refrigerant in a system that can bedescribed as open loop (Linde, Solvay or Claude cycles). In thesesystems, one uses the fact that the natural gas is present under highpressure. The natural gas is expanded in a valve, and during thisexpansion a small portion of the gas is liquefied. The liquid obtainedis collected, and the cold low pressure natural gas coming out of thevalve is conveyed to the low pressure pipe of the reduction station.These systems have the advantage of being relatively simple, but sincethe temperature obtained coming out of the valve depends on thecomposition of the gas and since the composition of natural gas isvariable, the gases liquefied with these systems are mainly heavy gasessuch as butane or propane but not methane. This gas liquefaction methodis also known as flashing.

All of the gas entering the pressure reduction station and passingthrough the valve or the turbine is cooled during the pressure drop thatoccurs. The gas still contains water and carbon dioxide at contents onthe order of around one hundred ppm or one percent. A condensationphenomenon can then occur during this expansion step, which is capableof causing the formation of ice (hydrates) that can block the pipes. Itis therefore necessary to treat the gas flow in order to prevent thewater and carbon dioxide contained in the natural gas from beingtransformed into ice in the pipes and thus causing problems in conveyingthe natural gas during its treatment in the pressure reduction stations.

The present invention aims in particular to provide means making itpossible, at the site of a pressure reduction station, to liquefy gas,particularly natural gas, by controlling the composition of theliquefied gas obtained. Advantageously, a device according to theinvention will make it possible. to recover energy of expansionresulting from the difference in pressure of the gas between the inletand the outlet of the pressure reduction station in order to produce aliquefied natural gas fraction while avoiding the formation of iceinside the pipes of these stations. The device will also preferably beeasy to use and have a simple design.

For this purpose, the present invention proposes a station for reducingthe pressure of a gas and for liquefying gas, particularly natural gas,comprising:

-   -   an expansion turbine,    -   means for recovering mechanical work produced during the        reduction of the pressure of the gas,    -   a refrigeration system comprising compression means, and    -   condensation means for liquefying gas.

According to the invention, this station moreover comprises means forrecovering heat produced by the compression means of the refrigerationsystem, which are associated with means for heating the gas upstream ofthe expansion turbine.

Such a station thus provides for integrating the heating of the naturalgas before its expansion and the cooling of the refrigerant while savinga significant amount of energy and/or gas for the manufacture of theliquefied (natural) gas.

A flow of (natural) gas in gaseous form is always maintained between ahigh pressure pipe and a low pressure pipe which are associated with apressure reduction station. Based on a volume of 100 m³ of natural gas,5 to 15 m³, for example, are transformed into liquefied natural gas.Work can here be recovered during the expansion between the two pressurelevels in order to be used later for transformation of a small portion(5 to 15%) of the (natural) gas into liquefied (natural) gas.

The heating of the gas occurs, for example, at the inlet of the pressurereduction station (that is to say upstream of the expansion turbine) byrecovery of the heat emitted by the compression means used forliquefying the gas. The gas going from the high pressure pipe to the lowpressure pipe is thus heated before entering the pressure reductionstation so that it is present at the outlet of said station with atemperature higher than the solidification point of the water.

In order to optimize the station described here and to recover a maximumamount of energy, it is provided that the gas under high pressure isfirst of all run into the expansion turbine and that subsequently,downstream of this turbine, a portion of the expanded gas is removed inorder to be sent to the condensation means. it is thus provided thatthese condensation means are supplied by a branch pipeline downstream ofthe expansion turbine.

According to a first embodiment, the station comprises a closed loopbetween the condensation means, the compression means and the means forheating the natural gas. This closed loop makes it possible to combine arefrigeration system (compressor and condenser) for the liquefaction ofthe gas with a heat exchanger, bringing about the thermal integrationbetween the reduction of the pressure of the gas and the production ofliquefied gas.

According to a second embodiment, the station comprises a first closedloop between the compression means, the condensation means and at leastone intermediate heat exchanger, as well as a second closed loop,possibly using a different heat transfer fluid from a heat transferfluid used in the first loop, between at least one intermediate heatexchanger and the means for heating the gas.

Proposed here, with these two embodiments, is a station with anintermediate system that can be likened to a closed loop, possiblydouble, making it possible to cool a fraction of the gas until itsliquefaction. The advantage of an independent closed loop system is thatit allows one to reach significantly lower temperatures inasmuch as itis not connected with the lowering of pressure achieved within thepressure reduction station. Thanks to this system, the composition ofthe liquefied gas hardly varies with respect to the entering gas, giventhat the change in state is obtained by direct cooling inside of a heatexchanger reserved for this operation, instead of the conventionalflashing system.

In a particular embodiment of a pressure reduction and liquefactionstation, the means for recovering mechanical work produced during thereduction of the pressure of the gas are associated with means forconverting mechanical work into electrical energy. In this embodiment,the means for recovering mechanical work produced during the reductionof the pressure of the gas can be mechanically coupled to an electricgenerator, and the compression means are advantageously driven by amotor supplied with electrical energy by the electric generator.

In another embodiment of a pressure reduction and. liquefaction station,the means for recovering mechanical work produced during the reductionof the pressure of the gas are mechanically associated with thecompression means. An auxiliary motor can optionally be provided fordriving the compression means.

Within such a station, one therefore has the integration of arefrigeration loop for liquefying gas and of preheating of the inlet ofthe expansion turbine.

The liquefied natural gas can be produced within a station according tothe invention from a refrigeration unit involving a refrigerant systemusing interchangeably nitrogen and/or a mixture of hydrocarbons.

A refrigeration system used in a station according to the invention canfor example comprise a heat exchanger and/or a condenser of the aluminumPFHE type.

In a particular embodiment, the refrigeration system comprisescompressors and/or radial flow expanders.

In another embodiment, the station according to the invention comprisesmeans for treatment of the water and carbon dioxide of the low pressurenatural gas by adsorption and/or absorption arranged upstream of themeans for condensation of the gas.

Details and advantages of the present invention will appear more clearlyfrom the description that follows given in reference to the appendeddiagrammatic drawing in which:

FIG. 1 is a very diagrammatic overview illustrating a station accordingto the present invention,

FIG. 2 is a more detailed diagrammatic view showing a first embodimentof the present invention,

FIG. 3 is a view similar to the view of FIG. 2, illustrating a secondembodiment of the invention,

FIG. 4 is a view similar to that of FIGS. 2 and 3, in the case of athird embodiment of the present invention, and

FIG. 5 is a view similar to that of FIGS. 2 to 4, in the case of afourth embodiment of the present invention.

FIG. 1 diagrammatically represents a gas pipeline 2 conveying a gas, forexample, natural gas composed of mostly methane, under high pressure,for example, on the order of 60 to 100 bars (generally in the presentapplication, the examples and the numerical values are illustrative andnon-limiting). A gas pressure reduction station, called PLD (Englishacronym for Pressure Let Down, or in French “baisse de pression”[lowering of pressure]) in FIG. 1, makes it possible to supply a pipe 4intended for supplying a domestic network or the like with gas (naturalgas, to re-use the preceding example) under low pressure, generally onthe order of a few bars.

A liquefied gas production unit 6 is associated with the pressurereduction station PLD. It is supplied with gas from the gas pipeline 2,downstream of the pressure reduction station PLD, goes through atreatment unit 8 that performs a treatment of the gas before it entersthe production unit 6 in order rid the gas of the impurities that aregenerally found in “raw” gas. Leaving the production unit 6, a liquefiednatural gas LNG is obtained which, for example, is stored in a storageunit (not illustrated in FIG. 1).

When gas is expanded in the pressure reduction station PLD, the gasgives up mechanical work WM. Proposed here is the recovery of all orpart of this work in some form, mechanical or electrical, for example,in order to supply the production unit 6 which requires energy in orderto switch the gas from its gaseous state to a liquid state. Inasmuch asthe energy recovered is not sufficient for the production of liquefiedgas, it is possible to supply the production unit with a supplementarysource of energy, for example, electrical energy diagrammaticallyrepresented by “WE” in FIG. 1. Finally, in the production unit 6, onegenerally has a compressor (not represented in FIG. 1) or another devicethat releases heat, represented simply by Q in FIG. 1. Proposed in anoriginal manner is the recovery of this quantity of heat Q in order toheat the gas entering the pressure reduction station PLD. Indeed, in thecourse of expansion, the expanded gas is cooled. It risks falling belowthe temperature of solidification of the water and thus leading toformation of ice which can lead to partial or complete obstruction ofthe corresponding pipeline. By heating the gas before the expansion, itis thus possible to limit the risks of icing and obstruction.

FIG. 2 shows in more detail a first embodiment of the inventionimplementing the overall scheme of FIG. 1.

In FIG. 2, as well as in the following figures, the references of FIG. 1are used again to designate similar elements.

One thus finds again in FIG. 2 a gas pipeline 2 which supplies apressure reduction station PLD in order to provide gas under lowerpressure in a pipe 4. Furthermore, a production unit 6 providesliquefied gas LNG.

In the pressure reduction station PLD, gas coming from the gas pipeline2 passes through pipes G2 and G3. It is heated in each of these pipes bya preheating device 10. Leaving these preheating devices, pipes G4 andG5 are collected in a pipe G6 which supplies an expansion turbine 12.Leaving the turbine, the gas is expanded and can rejoin the pipe 4directly through a pipe G7.

The production unit 6 essentially comprises a condenser 14. The gassupplying the production unit 6 is supplied from a branch G9 of the pipeG7 before coming to a valve 16 where an additional pressure reduction isachieved. The gas is conveyed through a pipe G10 to the treatment unit 8which performs a purification of the gas, for example, by absorption orpreferably by adsorption. The purified gas is conveyed through G11 tothe desuperheater 18 before being introduced through G12 into thecondenser 14. Leaving the latter, liquefied gas is obtained, whichpasses through a pipe L1 to a control valve 20 and then through L2 inorder to arrive at a storage device for liquefied natural gas LNG.

An interaction between the expansion turbine 12 of the pressurereduction station PLD and the production unit 6 is achieved here. Inthis embodiment of FIG. 2, the energy recovered during the expansion inthe station PLD is used in the form of electrical energy in theproduction unit 6, and the heat produced in the production unit 6 isused for heating the gas entering the station PLD, that is to sayupstream of the expansion turbine 12.

It is noted in FIG. 2 first that the turbine 12 is coupled to agenerator G. Thus, mechanical energy is recovered at the turbine 12 inorder to be converted into electrical energy. The electricity thusrecovered then supplies a motor M which drives three compressors C1, C2and C3 each forming a stage of a compression unit. In this way, anelectrical coupling is produced between the pressure reduction stationand the production unit.

In order to optimize the quantity of mechanical energy recovered at theturbine 12, both the gas intended for supplying the low pressure pipe 4and the gas intended for supplying the production unit 6, that is to saythe gas to be liquefied, are run into this turbine 12.

The thermal integration is achieved by a closed loop circuit which isdescribed hereafter. For this description, we propose then to follow therefrigerant fluid moving in this circuit. As a non-limiting example, thefluid used can be nitrogen or else a mixture of hydrocarbons.

The refrigerant fluid arrives in the compressor C1 through a pipe R1 andleaves it through a pipe R2. It then arrives in a first preheatingdevice 10 in order to heat gas that comes from the gas pipeline 2 andthat is intended for supplying the turbine 12 of the pressure reductionstation PLD. The fluid is then led through a pipe R3 to a cooler 22 inorder to achieve a control of the temperature of the refrigerant fluidbefore being sent to the compression unit through a pipe R4. The fluidis then compressed by the second compressor C2 and is then led throughR5 to the second preheating device 10 before being conveyed through R6to a second cooler 22 and reaching, through R7, a third compressionstage of the compression unit. A third cooler 22, connected to the thirdcompressor C3 through a pipe R8, makes it possible to control thetemperature of the fluid leaving the compression unit.

A pipe R9 brings the refrigerant fluid to a counterflow heat exchanger24, and is then led through R10 to an expander 26. The latter ismechanically connected to the motor M and to the compression unit.Leaving the expander 26, the fluid is then led (R11) to the condenser 14of the production unit 6 where it absorbs calories from the natural gasportion that one wishes to liquefy in order to obtain liquefied naturalgas (LNG). Leaving the condenser 14, the fluid is conveyed (R12) to thedesuperheater 18 before reaching, through R13, the counterflow heatexchanger 24 which is connected downstream to the first compressor C1 ofthe compression unit.

As emerges from this description, the refrigerant fluid is used toachieve a thermal integration between the production unit and thepressure reduction station by recovering in particular the caloriesreleased during the compression of the fluid in order to use them forheating the natural gas entering the pressure reduction station PLD.

Accessory elements of the refrigerant circuit are not described indetail here. One thus finds, for example, a tank 28 which is used in aconventional manner as expansion vessel for the refrigerant fluid.

FIG. 3 illustrates an embodiment variant which re-uses certainreferences of the preceding figures in order to designate similarelements. Compared to the embodiment of FIG. 2, another form of thermalintegration is achieved. It is proposed to have a closed loop ofpressurized water (or of another heat transfer fluid such as, forexample, a thermal oil) in order to recover the heat of compression andto transfer it upstream of the expansion turbine. An air cooler, forexample, can be placed on this line in order to adjust the coolingcapacity to the demand of the compression loop. A volumetric pump isused in order to allow the circulation of the heat transfer fluid(pressurized water), and an expansion vessel can in a conventionalmanner be integrated in this circuit.

One thus recognizes in FIG. 3 a refrigerant circuit between thecompression unit and its three compressors C1, C2 and C3 and theproduction unit 6 with its condenser 14. This circuit is simplified. Itpasses successively through the three stages of the compression unit,and after each stage, passes through a preheating device 10. Therefrigerant circuit then passes through the counterflow heat exchanger24 before going into the expander 26 and then into the condenser 14,again passing through the counterflow heat exchanger 24 and coming backto the first compression stage and its compressor C1.

The main difference from the first embodiment of FIG. 2 is that thepreheating devices 10 do not directly transfer the calories extractedfrom the compression stages to the natural gas but rather they transferthem to another heat transfer fluid such as, for example, pressurizedwater. A second refrigerant circuit is thus produced, which passes inparallel through the three preheating devices 10 in order to supply apreheating device 110 which transfers the calories coming from thecompression stages to the natural gas entering the station PLD. Thesepreheating devices 10 thus form intermediate heat exchangers. Betweenthe preheating devices 10 and the preheating device 110, one notes thepresence of a volumetric pump 142 making it possible to circulate theheat transfer fluid in the corresponding circuit, as well as a cooler122 for controlling the temperature of the heat transfer fluid in thiscircuit. In a conventional manner for the person skilled in the art, anexpansion vessel 144 is advantageously integrated in this refrigerantcircuit.

As for FIG. 4, it illustrates a simplified version of the firstembodiment illustrated in FIG. 2. Here also, as is generally the case inthe present application, the references already used are re-used fordesignating similar elements in order to simplify reading comprehension.

In this simplified embodiment, one notes that the compression unit onlyhas a single stage with a single compressor C. The natural gas is thenheated within a single preheating device 10 which allows a directexchange of the calories coming from the compressor with the natural gasentering the station PLD, upstream of the expansion turbine 12.

In this embodiment, the refrigerant circuit uses, for example, a mixtureof hydrocarbons and nitrogen as heat transfer fluid. The latter iscompressed by the compressor C driven by the electric motor M(electrically coupled to the generator G of the turbine 12 of thestation PLD. The fluid then cooled in contact with the natural gas inthe preheating device 10 at the entrance of the turbine 12 (it isappropriate to note that one could here also provide another refrigerantcircuit between the preheating device 10 and the natural gas as in thepreceding figure).

A cooler 22 or (air cooler) can be introduced into the circuit in orderto adjust the cooling capacity to the demand of the compression loop.The heat transfer fluid is then sent through a heat exchanger 214, forexample, of type PHFE (English acronym for Plate Fin Heat Exchanger orin French “échangeur de chaleur à plaques et ailettes” [plate and finheat exchanger]), where it is cooled and condensed during a first pass.It is then expanded through a valve 246 where, by Joule-Thompson effect,it partially vaporizes, again causing a lowering of its temperature. Itagain passes (2^(nd) pass) through the heat exchanger 214 and vaporizesand is heated in contact with the natural gas to be liquefied and withthe refrigerant mixture to be condensed. After this second pass, leavingthe heat exchanger 214, the heat transfer fluid (mixture of hydrocarbonsand nitrogen, for example) returns to the compressor C.

In the embodiment of FIG. 5, compared to the embodiments of thepreceding figures, between the pressure reduction station and theproduction unit, a mechanical integration (FIG. 5) is achieved insteadof an electrical integration (FIGS. 2 to 4).

Indeed, whereas in the embodiment of FIG. 2, the turbine 12 drives agenerator G which produces electricity which is consumed in a motor M,it is proposed in FIG. 5 to mechanically connect the turbine 12 with thecompressors C1, C2 and C3 of the compression unit of the production unit6.

It seems unnecessary to describe here the different elements of thepressure reduction station that are similar to those represented in FIG.2. Likewise, one finds again a similar refrigerant circuit for producingboth the liquefied gas production unit and the thermal integration ofthis production unit with the pressure reduction station.

Also represented in this FIG. 5 is a motor M which is used here asadditional energy source (corresponds to WE in FIG. 1) in order toadjust the power necessary for the liquefied gas production unit withthe power delivered at the site of the pressure reduction station.

As a purely illustrative example, it is possible to provide, forexample, in the various embodiments described, that the quantity(weight) of gas passing through the liquefied gas production unit 6 ison the order of 5 to 20% of the quantity (weight) of gas passing throughthe pressure reduction station PLD, the rest of the gas (80 to 95%)supplying the pipe 4.

The systems described above allow complete control of the production ofliquefied natural gas. The composition of this gas can be controlled. Itdoes not depend on the difference in pressure within the pressurereduction station.

Furthermore, the preheating of the gas entering the pressure reductionstation makes it possible to prevent problems of icing and obstructionof the pipeline.

Energy recovery takes place at the pressure reduction station, and moreprecisely at its expansion turbine. This recovery is optimized by havingthe whole gas flow pass through this turbine, that is to say the gaswhich is intended for being expanded in gaseous form as well as the gasintended for being liquefied.

The present invention is not limited to the preferred embodimentsdescribed above as non-limiting examples. It also relates to theembodiment variants accessible to the person skilled in the art withinthe scope of the claims hereafter.

1-11. (canceled)
 12. A station for reducing the pressure (PLD) of a gasand for liquefying the gas, comprising: an expansion turbine (12); meansfor recovering mechanical work (WM) produced during reduction ofpressure of the gas; a refrigeration system comprising compression means(C1, C2, C3); condensation means (14) for liquefying the gas; and meansfor recovering heat (Q) produced by the compression means (C1, C2, C3;C) of the refrigeration system; the compression means associated withmeans (10; 110) for heating the gas upstream of the expansion turbine(12).
 13. The station according to claim 1, further comprising a branchpipeline (G9) downstream of the expansion turbine (12) for supplying thecondensation means (14).
 14. The station according to claim 1, whereinsaid station comprises a closed loop between the condensation means(14), the compression means (C1, C2, C3; C) and the heating means (10)for the gas.
 15. The station according to claim 1, wherein said stationcomprises a first closed loop between the compression means (C1, C2,C3), the condensation means (14) and at least one intermediate heatexchanger (10); and a second closed loop, between the at least oneintermediate heat exchanger (10) and the heating means (110) for thegas.
 16. The station according to claim 15, wherein the first closedloop comprises a first heat transfer fluid, and the second closed loopcomprises a second heat transfer fluid different from said first heattransfer fluid.
 17. The station according to claim 1, further comprisingmeans for converting (G) mechanical work into electrical energy, saidconverting means associated with the means for recovering mechanicalwork (WM) produced during the reduction of the pressure of the gas. 18.The station according to claim 17, wherein the converting means (G)comprises an electrical generator mechanically coupled to the means forrecovering mechanical work (WM), and further comprising a motor (M)supplied with electrical energy by the electric generator for drivingthe compression means (C1, C2, C3).
 19. The station according to claim1, wherein the means for recovering mechanical work (WM) produced duringthe reduction of the pressure of the gas is mechanically connected tothe compression means (C1, C2, C3; C).
 20. The station according toclaim 19, further comprising an auxiliary motor (M) for driving thecompression means (C1, C2, C3).
 21. The station according to claim 1,wherein the refrigeration system comprises a refrigerant selected fromthe group consisting of nitrogen, a mixture of hydrocarbons, andnitrogen and a mixture of hydrocarbons.
 22. The station according toclaim 1, further comprising compressors and/or radial flow expanders.23. The station according to claim 1, further comprising means fortreatment (8, 36) of the gas by at least one of adsorption, adsorptionarranged upstream of the condensation means (14), and adsorption andabsorption arranged upstream of the condensation means.
 24. The stationaccording to claim 1, wherein the gas comprises natural gas.